Treatment of steel



Patented May 12, 1936 UNITED I STATES TREATMENT OF STEEL Morris G. Fowler and Lyle M. Barker, Clarkdale,

., assignors,

by mesne assignments, to

Phelps Dodge Corporation, New York, N. Y., a

corporation of New No Drawing.

York

Application October 6, 1932,

Serial No. 636,577

Claims.

This invention relates to the production of steel products and has for an object the provision of an improved method of forming copper steel products. More particularly, the invention contemplates the provision of a process for preventing surface checking of copper steel during forging or rolling.

Copper steels have long had the reputation of being red short and cold short, the reason being that copper magnified the ill effects of oxygen and sulphur present in the steel. Copper steels in which the sulphur and oxygen contents have been reduced sufiiciently low may be forged or rolled easily, but, after forging or rolling under usual conditions, the finished shapes have had very poor surfaces due to surface checks or cracks. Also, due to penetration of scale into these checks or cracks, the scale is very adherent, can not be removed readily, and may be rolled into the steel.

The severity of surface checking of copper steels varies with the copper content, the higher the copper content the worse the checking. A copper content of about 2% or more, in steel, will cause severe surface checkingwhen the steel is forged or rolled under usual conditions of heating and working. A copper content of 1.5% causes only a moderate amount of surface checking, and a copper content of less than 1% causes only very slight surface checking. A copper-free steel scales easily but under comparable conditions shows little or no checking.

The surface checks on copper steels have the eifect of reducing the apparent strength of the steels in that they act as nicks" or notches and cause premature failure or rupture when the steel is strained beyond its elastic limit, either hot or cold. This is especially serious in steel that must be bent to shape, after being forged or rolled under usual conditions, since the checks are the beginnings of tears in the metal. and result in serious weakening or complete failure at the bend.

We have demonstrated that the surface checks on copper steels are not the result of red shortness or cold shortness by grinding or machining oil the surface of a forged copper steel bar, below the depth of penetration of the checks, and then testing the bar. A bar so treated will bend hot or cold as well as a bar of copper-free steel of equal hardness, thus showing that the surface checks are not the result of red shortness or cold shortness.

Our researches indicate that surface checking of copper steel probably can be attributed to the existence of a film of liquid copper on the surface of the steel, resulting from selective oxidation of iron. When a copper steel is heated in the pres ence of a gas that will oxidize iron, the iron is selectively oxidized to iron oxide which forms scale and the copper is concentrated on the steel surface, the degree of concentration depending on the temperature and amount of scaling. The maximum concentration seems to take place at a temperature near the melting point of copper.

The following is an example of scaling tests made to determine the effect of temperature on the concentration of copper on a'steel surface during scaling. Pieces of copper steel containing 2% copper were heated in an electric muflie furnace, with fairly free access of air, for times and temperatures indicated ,below. The pieces were cooled in air, and the scale was cracked off and analyzed for iron and coppe The above figures appear to indicate that a very considerable concentration of copper on the steel surface takes place during scale formation while the steel is being heated to forging temperatures. Once the copper film is formed on the surface of the steel, further scale formation apparently takes place by transfer of iron through the copper plate. The copper tends to alloy with the iron and the iron is oxidized away from the outer copper surface. We probably have the following conditions when copper steel is being treated in an oxidizing atmosphere: starting with the body of the iron and proceeding to the exterior, we have-Fe; Cu-rich Fe; Fe-rich Cu grading into Fe lean-Oz rich Cu; and finally Fe oxide scale, Under these conditions, and especially above the melting point of copper, the copper becomes an active corrosion agent dissolving iron and expediting its oxidation. The attack of the iron bythe copper proceeds relatively uniformly over the iron surfaces, that is, without apparent relation to the Fe grain boundaries. Likewise, the liquid copper appears capable of readily wetting the iron surface, when the temperature is above the melting point of copper.

The above described precipitation of copper on the surface of copper steel, together with the attack of the iron by the combination of oxygen and copper does not in itself effect surface checking, a surface plating of copper being the result.

However, if the steel is worked at a temperature at which the copper film is liquid, serious surface checking will result. This probably is .due to the forcing of the copper between the iron grains or its capillary penetration into minute crevices opened up by the working.

We have discovered that objectionable surface checking of copper steel may be avoided through control of the character of the surface of the steel at the commencement of the shaping operation. Thus, for example, we have discovered that objectionable surface checking may be avoided by so preparing the steel for forging or rolling as to preclude the existence of surface films of liquid copper.

In accordance with our invention, copper steel may be prepared for shaping, as by forging or rolling, as follows:

(1) By heating to suitable shaping temperatures under such conditions as 'to prevent scale formation;

(2) By maintaining or establishing non-liquid surfaces through temperature regulation;

(3) By eliminating scale formed during oxidizing'heat treatments; and

(4) By incorporating in the copper steel, prior to casting into ingots, an alloying element which reduces the tendency of copper to liquefy at temperatures above the melting point of copper.

By preventing the formation of scale, selective oxidation of the iron in the surface portions of the steel is avoided and concentration of metallic copper on the surface of the steel is prevented. Scale formation may be prevented by heating the steel in a non-oxidizing atmosphere or in a reducing atmosphere. Scale-free copper steel, at temperatures considerably higher than the melting point of copper, may be forged or rolled into finished shapes with access of air without serious surface checking resulting. Thus, for example, we have produced finished shapes with only very slight surface checking by forging scale-free copper steel at temperatures of 1100 C. to 1150 C. Immediately before forging the steel had been heated to the forging temperature in an atmosphere of hydrogen. The surfaces of similar shapes which had been forged from steel heated to similar temperatures in oxidizing atmospheres were very badly checked. Bars forged from the scale-free steel could be bent through 180 without any apparent rupturing or tearing resulting, while bars forged from the steel heated in the oxidizing atmospheres were torn at the bend when bent at an angle of about 90.

The slight surface checking of scale-free steel worked at temperatures above the melting point of copper results from scale formed as a result of oxidation which takes place while the steel is being worked. Surface checking in finished shapes formed from copper steel heated under such conditions as to prevent scale formation may be substantially completely prevented by controlling the heating operation to develop a temperature below about 1100 C. and preferably below the melting point of copper or. by cooling the highly heated scale-free steel to a suitable temperature below 1100 C. before forging or rolling.

Regulation of the temperatures of ingots and billets to maintain or establish non-liquid surfaces may be accomplished by heating, prior to forging, to temperatures below the melting point of copper or by heating to high temperatures and cooling to temperatures below the melting point of copper prior to forging. We have produced finished shapes substantially free from surface checks by forging steel heated to temperatures of 1050 C. to 1100 C. in oxidizing atmospheres and by forging steel heated to 1150 C. in oxidizing atmospheres and cooled to about 1050 C. in air and in water immediately before forging.

Similar shapes produced from steel heated to temperatures of 1125 C. to 1150 C. in oxidizing atmospheres and forged without cooling were badly checked. The aforementioned temperature of 1100" C. was the temperature at which the steel was removed from the heating furnace. When forging was commenced, the surface temperature had dropped to about 1075 C.

Elimination of scale may be accomplished by shaving, grinding, or machining or by heating the steel in a strongly reducing atmosphere to reduce the iron oxide of the scale. The metallic iron produced by reduction of the iron oxide forms an alloy with the metallic copper which has a higher melting point than the copper and which freezes and forms a solid metallic surface on the steel. Elimination of the scale mechanically or through the use of reducing agents permits the production of shapes substantially free from surface checks from steel at temperatures as high as may be required for forging, provided, of course, that the time required for the shaping operation is not so great as to permit excessive scale formation before its completion.

We have employed elements such as aluminum, titanium and zirconium for developing resistance to surface checking. The addition of such elements to the molten steel before casting in ingots may be made for the combined purpose of deoxidizing the steel and forming an alloy with the steel. The addition of an amount of the element such as will permit the formation of an alloy containing a fraction .of one percent of the element is satisfactory.-

In rolling or forging copper steel in accordance with our invention, care should be exercised to avoid the existence of a film of liquid copper on the steel'at the commencement of the forging or rolling operation and to avoid the production of, liquid copper during the forging or rolling operation. Thus, ingots or billets having scale formed thereon should be forged or rolled at temperatures below the melting point of copper. If such ingots or billets have been heated they should be cooled by standing in the air or by means of a water spray, steam or air blast to a suitable temperature at which copper can not exist in liquid form. If the forging or rolling of copper steel having scale thereon is to be conducted at temperatures above the melting point of copper, the scale should be removed prior to the commencement of the forging or rolling operation by scraping, machining or planing or by heating in a reducing atmosphere to reduce the iron oxide of the scale. Y

Forging or rolling operations for converting scale-free ingots or billets into finished shapes should not be conducted at unnecessarily high temperatures. The initial temperature selected in each case should be such that the steel will not remain above the melting point of copper for a period of time which will permit sufilcient oxidation to cause checking. This precaution must be observed particularly in the forging or rolling of large ingots when relatively long periods of working are required. In general, any initial forging or rolling temperature that is suitable to the particular steel may be employed in the forging or rolling of scale-free steel.

When an ingot or billet is to be reduced to a finished shape by forging or rolling in two or more stages, any suitable heat treatment may be employed for establishing any suitable forging or rolling temperature for all stages except the final stage. The temperature and/or manner of heating for the final stage should be such as to preclude the existence of liquid copper when working is commenced or the production of liquid copper during working, or scale appearing on the steel at the end of the stage preceding the final stage should be removed mechanically before the final working stage is commenced. Thus, for example, a preliminary mechanical reduction of steel having scale thereon at a temperature above the melting point of copper, may be followed by chemical reduction of the iron oxide of the scale or by lowering of the temperature to a point below the melting point of copper, or the scale may be removed by shaving, grinding or machining,

before the commencement of the .final mechanical reducing operation.

We claim:

1. The method of mechanically shaping copper steel which comprises commencing the shaping operation when the surface of the steel is at a temperature above the melting point of copper at which the copper contained therein is in a. non-liquid condition, thereby to inhibit surface checking.

2. The method of mechanically shaping copper steel which comprises commencing the shaping operation when the surface of the steel is at a temperature of about 1075 0., thereby to inhibit surface checking.

3. In a. process for mechanically shaping copper steel involving the heating of relatively cold steel to a suitable shaping temperature in an oxidizing atmosphere, and forging or rolling of the steel while hot, the improvement which comprises discontinuing the heating operation when a surface temperature of a bout 1100 C. has been established, and commencing the shaping operation when the surface temperature has decreased to a point 'below the melting point of copper, thereby to inhibit surface checking.

4. In a process for mechanically shaping copper steel involving the heating of relatively cold steel to a suitable shaping temperature in an oxidizing atmosphere, and forging or rolling of the steel while hot, the improvement which comprises discontinuing the heating operation when a. surface temperature of about 1100 C. has been established, and commencing the shaping operation when the surface temperature has decreased to about 1075 0., thereby to inhibit surface checking.

5. In a process for mechanically shaping copper steel involving the heating of relatively cold steel to a temperature above about 1100 C. in an oxidizing atmosphere, and forging or rolling of the steel while hot, the improvement which comprises cooling the heated steel to a temperature below the melting point of copper and eliminating scale from the surface of the cooled steel prior to shaping, thereby to inhibit surface checking.

6. In a process for mechanically shaping copper steel involving the preliminary heating of the steel to a temperature above 1100 C. and forging or rolling of the steel while hot, the improvement which comprises conducting the preliminary heating of the steel in a non-oxidizing atmosphere, and cooling the heated steel to a temperature below the melting point of copper prior to shaping, thereby to inhibit surface check- 7. In a process for mechanically shaping copper steel involving the preliminary heating of the steel to a temperature above 1100 C., and forging or rolling of the steel while hot, the improvement which comprises conducting the preliminary heating of the steel in a reducing atmosphere, and cooling the heated steel to a temperature below the melting point of copper prior to shaping, thereby to inhibit surface checking.

8. The method of treating copper steel which comprises heating the steel to a temperature not higher than about 1100 C. and not lower than about 1050 C., and subjecting the resulting heated product to a mechanical shaping operation, thereby to form a copper steel shape substantially free from surface checks.

9. The method of shaping copper steel without substantial surface checking which comprises heating the steel to a temperature above the melting point of copper under such conditions as to substantially prevent scale formation, and subjecting the heated steel to a mechanical shaping operation at a temperature above 1050" C.

10. The method of shaping copper steel without substantial surface checking resulting which comprises heating the steel to a temperature above the melting point of copper under such conditions as to substantially prevent scale formation, and subjecting the heated steel to a mechanical shaping operation at a temperature above the melting point of copper.

11. The method of shaping copper steel without substantial surface checking resulting which comprises heating the steel to a temperature above the melting point of copper, removing scale therefrom, leaving a substantially scale-free product, and subjecting the substantially scalefree product to mechanical shaping operation, the scale elimination being conducted and the shaping operation being commenced while the steel is at a temperature above the melting point of copper.

12. The method of shaping copper steel containing more than 1% of copper without substantial surface checking resulting which comprises heating the steel to a temperature above the melting point of copper under such conditions as to substantially prevent scale formation, and subjecting the heated steel to a mechanical shaping operation at a temperature above 1050 C.

13. The method of shaping copper steel containing more than 1% of copper without substantial surface checking resulting which comprises heating the steel to a temperature above the melting point of copper, removingscale therefrom, leaving a substantially scale-free product, and subjecting the substantially scale free product to a mechanical shaping operation, the scale elimination being conducted and the shaping operation being commenced while the steel is at a temperature above the melting point of copper.

14. The method of mechanically shaping copper steel which comprises commencing the shaping operation when the surface of the steel is at a temperature above 1050 C. but below the melting point of copper, thereby to inhibit surface checking.

15. The method of shaping copper steel without substantial surface checking resulting which comprises heating the steel to a temperature above the melting point of copper but not exceeding about 1150 C., cooling the heated steel to a temperature below the melting point of copper but not substantially lower than 1050 C., and subjecting the cooled steel to a mechanical shaping operation.

MORRIS G. FOWLER.

LYLE M. BARKER. 

